Regarding capturing of carbon dioxide, carbon dioxide capture and storage technology has recently received attention as an effective measure against global warming issues concerned on a global mass scale. In particular, a method of capturing the carbon dioxide by using an aqueous solution has been studied in association with a thermal power plant and a process exhaust gas. For example, a carbon dioxide capturing apparatus is known which includes an absorption tower configured to generate a rich liquid by causing an absorption liquid to absorb a gas containing carbon dioxide, and a regeneration tower configured to heat the rich liquid discharged from the absorption tower to release the carbon dioxide and steam, separate the carbon dioxide from the steam, and return a generated lean liquid to the absorption tower. In this carbon dioxide capturing apparatus, the cold rich liquid is preheated with the hot lean liquid by a regenerative heat exchanger and is fed to the regeneration tower, so that an amount of energy required for releasing the carbon dioxide is reduced.
However, since the rich liquid and the lean liquid flow in liquid phases through the regenerative heat exchanger, heat transfer characteristics between these absorption liquids are low. When a temperature of the rich liquid is elevated close to an operation temperature of the regeneration tower by the regenerative heat exchanger for the purpose of reducing an amount of energy input at the regeneration tower, a difference in temperature between the rich liquid and the lean liquid becomes small in the vicinity of an outlet of the regenerative heat exchanger. Specifically, driving force for transferring heat from the lean liquid to the rich liquid becomes small in the vicinity of the outlet of the regenerative heat exchanger. Therefore, a large regenerative heat exchanger is required for securing a wide heating area. On the contrary, when the difference in temperature between the rich liquid and the lean liquid in the vicinity of the outlet of the regenerative heat exchanger is made large, a temperature elevation of the rich liquid at the regeneration tower becomes large, which increases the amount of energy input at the regeneration tower.
In order to solve such problems, a regenerative heat exchanger of plate type is used which is compact and has high heat transfer characteristics. It can also be conceivable to set a pressure of the rich liquid side to be low so as to generate steam (water vapor) and a carbon dioxide gas from the rich liquid while its temperature is elevated toward the outlet of the regenerative heat exchanger. In this case, extra heat recovery from the lean liquid can be achieved by latent heat of vaporization during the steam generation and heat of dissociation during the generation of the carbon dioxide gas from the rich liquid. Therefore, even when the temperature of the rich liquid is not elevated close to the operation temperature of the regeneration tower, the amount of energy input at the regeneration tower can be suppressed. Since the difference in temperature between the rich liquid and the lean liquid does not have to be made small, an increase of the heating area in the regenerative heat exchanger can be suppressed.
However, when the rich liquid in the regenerative heat exchanger of plate type becomes a two phase flow of a gas and a liquid in which the liquid and the gas are mixed, their flow rates in a plurality of channels between plates become uneven. When the ratio of the gas component in the two phase flow increases, heat transfer planes of the regenerative heat exchanger of plate type are dried. As a result, heat transfer performance of the regenerative heat exchanger deteriorates and its operation becomes unstable. On the other hand, when the temperature elevation of the rich liquid at the regenerative heat exchanger is made small to suppress the generation of the gas, the heat recovery from the lean liquid is not sufficient, causing an effect of reducing the amount of energy input at the regeneration tower to be small.